Conventionally, an air-fuel ratio control apparatus has been widely known, which comprises a three-way catalytic converter (53) disposed in an exhaust passage of an internal combustion engine, and an upstream-side air-fuel ratio sensor (67) and a downstream-side air-fuel ratio sensor (68) disposed upstream and downstream of the three-way catalytic converter (53), respectively, as shown in FIG. 1.
The air-fuel ratio control apparatus calculates an air-fuel ratio feedback amount based on an output of the upstream-side air-fuel ratio sensor and an output of the downstream-side air-fuel ratio sensor, and feedback controls an air-fuel ratio of a mixture supplied to the engine (an air-fuel ratio of the engine) with the air-fuel ratio feedback amount, so that the air-fuel ratio of the engine coincides with the stoichiometric air-fuel ratio. Further, an air-fuel ratio control apparatus has also been widely known, which calculates “an air-fuel ratio feedback amount to make the air-fuel ratio of the engine coincide with the stoichiometric air-fuel ratio” based only on the output of the upstream-side air-fuel ratio sensor, and feedback controls the air-fuel ratio of the engine with the feedback amount. The air-fuel ratio feedback amount used in those air-fuel ratio control apparatuses is a control amount commonly used for all of the cylinders.
Meanwhile, an electronic control fuel injection type internal combustion engine, typically, comprises at least one fuel injector (39) in each of the cylinders or in each of the intake ports, each communicating with each of the cylinders. Accordingly, when a characteristic of an injector for a specific cylinder becomes “a characteristic that the injector injects fuel by an amount which is larger (more excessive) than an instructed fuel injection amount”, only an air-fuel ratio of a mixture supplied to the specific cylinder (air-fuel-ratio-of-the-specific-cylinder) changes toward extremely richer side. That is, an imbalance among air-fuel ratios of the cylinders (a variation in air-fuel ratios among the cylinders, air-fuel ratio non-uniformity among the cylinders) becomes large. In other words, there arises an imbalance among “the individual-cylinder-air-fuel-ratios”, each of which is an air-fuel ratio of a mixture supplied to each of the cylinders.
In this case, an average of the air-fuel ratios of the mixtures supplied to the entire engine becomes an air-fuel ratio richer (smaller) than a stoichiometric air-fuel ratio. Accordingly, the feedback amount common to all of the cylinders causes the air-fuel ratio of the specific cylinder to change to a leaner (larger) air-fuel ratio so that the air-fuel ratio of the specific cylinder is made closer to the stoichiometric air-fuel ratio, and at the same time, the feedback amount causes the air-fuel ratios of the other cylinders to change to a leaner (larger) air-fuel ratios so that the air-fuel ratios of the other cylinders are made deviate more from the stoichiometric air-fuel ratio. As a result, the average of the air-fuel ratios of the mixtures supplied to the entire engine is made roughly equal to the stoichiometric air-fuel ratio.
However, the air-fuel ratio of the specific cylinder is still richer (smaller) than the stoichiometric air-fuel ratio, and the air-fuel ratios of the other cylinders are still leaner (larger) than the stoichiometric air-fuel ratio, and therefore, a combustion condition of the mixture in each of the cylinders is different from a perfect combustion. As a result, an amount of emissions (an amount of an unburnt substances and/or an amount of nitrogen oxides) discharged from each of the cylinders increases. Accordingly, although the average of the air-fuel ratios of the mixtures supplied to the engine coincides with the stoichiometric air-fuel ratio, the three-way catalytic converter can not purify the increased emission, and thus, there is a possibility that the emission becomes worse.
It is therefore important to detect whether or not the air-fuel ratio non-uniformity among cylinders becomes excessively large (an air-fuel ratio imbalance among cylinders is occurring), because an appropriate measure can be taken in order not to worsen the emissions. It should be noted that the air-fuel ratio imbalance among cylinders occurs when a characteristic of an injector of a specific cylinder becomes “a characteristic that the injector injects fuel by an amount which is excessively smaller than the instructed fuel injection amount”.
One of such conventional apparatuses that determine whether or not the air-fuel ratio imbalance among cylinders is occurring obtains a trajectory length of an output value (output signal) of an air-fuel ratio sensor (the above mentioned upstream-side air-fuel ratio sensor 67) disposed at an exhaust-gas-aggregated-portion to which exhaust gases from a plurality of cylinders aggregate, compares the trajectory length with “a reference value varying in accordance with an engine rotational speed”, and that determines whether or not the air-fuel ratio imbalance among cylinders is occurring based on the comparison result (refer to, for example, U.S. Pat. No. 7,152,594).
It should be noted that, in the present specification, “an occurrence of the air-fuel ratio imbalance among cylinders” means an occurrence of a state in which a difference between individual cylinder air-fuel ratios (individual cylinder air-fuel-ratio difference) is greater than or equal to a tolerable value, and in other words, it means an occurrence of an excessive air-fuel ratio imbalance among cylinders which causes the unburnt substances and/or the nitrogen oxides to exceed predetermined tolerable values. “Determining (judging) whether or not the air-fuel ratio imbalance among cylinders is occurring” can be simply referred to as “a determination of an air-fuel ratio imbalance among cylinders, or an imbalance determination”. Furthermore, a cylinder to which an air-fuel mixture is supplied, whose air-fuel ratio is different (or apart) from an air-fuel ratio (e.g., the stoichiometric air-fuel ratio) of an air-fuel mixture supplied to each of the other cylinders, is also referred to as “an imbalance cylinder”. The air-fuel mixture supplied to the imbalance cylinder is also referred to as “an imbalance cylinder air-fuel ratio”. Each of the other cylinders (each cylinder other than the imbalance cylinder) is also referred to as “a normal cylinder” or “an un-imbalance cylinder”. The air-fuel mixture supplied to the normal cylinder is also referred to as “a normal cylinder air-fuel ratio” or “an un-imbalance cylinder air-fuel ratio”.
In addition, a value which increases as an absolute value of the individual cylinder air-fuel-ratio difference (an absolute value of a difference between the imbalance cylinder air-fuel ratio and the normal cylinder air-fuel ratio) increases, just like the trajectory length of the output value of the air-fuel ratio sensor, may be referred to as an air-fuel-ratio-variation-indicative-value. That is, the air-fuel-ratio-variation-indicative-value is “a value which is obtained based on the output value of the air-fuel ratio sensor” in such a manner that its absolute value becomes greater as a variation of the air-fuel ratio of the exhaust gas arriving at the above mentioned air-fuel ratio sensor becomes greater. Further, a value, which becomes greater as an absolute value of the air-fuel-ratio-variation-indicative-value becomes greater, and which is obtained based on the air-fuel-ratio-variation-indicative-value, can be referred to as “a parameter for imbalance determination (judgment)”. The parameter for imbalance determination is compared with a threshold value for imbalance determination in order to perform the imbalance determination.